Fix sjl/learnvimscriptthehardway#63
`:help silent!` throws an error E149: Sorry, no help for silent!, however, `:help :silent` opens up the expected help file.
    
        | author | nirenjan <nirenjan@gmail.com> | 
    
        | date | Mon, 29 Jul 2013 18:08:25 -0700 | 
    
    
        | parents | 56683eb09ecc | 
    
        | children | 019d24c8e88e | 
    
        | branches/tags | (none) | 
    
        | files | chapters/53.markdown | 
Changes
    
--- a/chapters/53.markdown	Thu Apr 04 01:38:14 2013 -0400
+++ b/chapters/53.markdown	Mon Jul 29 18:08:25 2013 -0700
@@ -256,5 +256,5 @@
 
 Suppose you wanted to programatically force a reload of an autoload file Vim has
 already loaded, without bothering the user.  How might you do this?  You may
-want to read `:help silent!`.  Please don't ever do this in real life.
+want to read `:help :silent`.  Please don't ever do this in real life.